Q: how to connect a smart watch to iphone6

That would really depend on the make and model of the watch. Generally, you follow the manufacturer's instructions.

What kind of watch is it?

Hi

To pair Apple Watch with your iPhone and set it up, follow the instructions here:

• Set up your Apple Watch - Apple Support

For guidance on connecting and setting up any other watch, refer to the manufacturer's user guide. General instructions for pairing Bluetooth accessories to your iPhone are available here:

• Set up and use third-party Bluetooth accessories - Apple Support